This invention relates to fiber reinforced plastics. More particularly, this invention relates to oriented-fiber thermoplastics and methods for continuously preparing oriented-fiber thermoplastic stock material.
It is well known that plastic materials can be reinforced by imbedding in the plastic matrix randomly oriented strands of fibers in the form of cut staple, i.e. fibers that are cut into short lengths of 1 to 5 inches. While it is recognized that fiber reinforced plastics have physical and mechanical properties that make them particularly well suited for use where high strength, low weight and good chemical resistance are particularly important, full commercial exploitation of such materials has not been achieved because of a lack of a completely economical method of preparing these materials.
Perhaps, one of the simplest techniques for forming reinforced plastic matrix is to first coat the fiber with a thermoset resin, such as an epoxy resin, and thereafter cut the resin coated fiber into staple lengths. This technique has the disadvantage in that it is generally difficult to control the amount of fiber loading in the composite over a wide range. Since the viscosity of uncured thermoset resin is generally low, the fiber content of the resulting composite material tends to be high (e.g. usually greater than 40 volume %). Nonetheless fiber reinforced thermoset resins are commercially available.
Other reinforced thermoplastics would appear to offer certain more attractive properties from a commercial point of view than thermoset resins; however, numerous difficulties in fabricating such materials have been encountered.
Among the approaches employed in fabricating composites of fibers and thermoplastic resins is that described in British Pat. No. 1,228,573. That patent teaches making a composite from reinforcing filaments of staple length by arranging them either randomly or in a particular pattern along with thermoplastic filaments or films and coalescing the entire mass in a mold under elevated temperatures and pressure to produce the composite material. This process suffers from being uneconomical especially when considering all the attendant problems of quality control and uniformity of products that batch processing entails. Moreover, short chopped fibers are not easily wet by thermoplastic resins and therefore do not impart the same strength to the composite as do continuous fibers. Continuous fibers can be considered to be fibers that have an aspect ratio, i.e., a ratio of length to diameter, greater than 250. Additionally chopped fibers can only be randomly oriented within the matrix while continuous fibers can be arranged in a specific direction in the matrix.
A second technique for applying a thermoplastic coating to a fiber involves coating a continuous fiber, for example, with a polymer melt. Such a technique, of course requires, in the first instance, that the polymer be thermally stable. In addition, however, the viscosity of the polymer melt is usually very high and it becomes difficult to achieve good wetting and fiber penetration with the polymer melt.
Another approach to applying or impregnating a fiber with a thermoplastic resin employs the use of a fluidized bed of the powdered polymer. This technique avoids the thermal degradation problems present in the melt technique since only the polymer powder which comes in contact with the fiber is brought to the melt temperature. While minimizing the thermal degradation problems, however, the problems of wetting and penetration remain.
In yet another method described, in British Pat. No. 1,227,756, a solution of a polymer is used for coating reinforcing filaments. The solvent subsequently is removed and the filaments are then cut into stable lengths and blended with additional matrix material for compounding.
As indicated in British Pat. No. 1,227,756, the chopped fibers can be blended with matrix material and molded into a desired shape. Such blending, of course, does not permit specific orientation of the fibers in the matrix material.
Consequently, despite the advances made by the foregoing techniques in fiber reinforcing thermoplastics, there is an ever increasing need for new and improved techniques in forming such materials.